The present invention relates to a process for the manufacture of oriented hollow plastic articles which possess a practically constant wall thickness in the stretched zones and which, consequently, possess improved mechanical properties which are the same at all points.
Conventional processes have been known for a long time for the manufacture of hollow articles, such as bottles and the like, starting from plastic. These processes generally make use of the technique of blow-molding a preform which can be either a rough shape which has been injection-molded beforehand, or simply a tubular portion of plastic extruded directly into the blow-mold. Taking account of the relatively low rate of production achieved when employing injection presses, it is generally preferred to resort to the second technique which is commonly called extrusion blow-molding.
More recently, it has been found that the temperature of the rough shape at the instant when it is blow-molded into its final shape has a great influence on the mechanical properties of the articles when they have been released from the mold after cooling.
It has been noted, in fact, that no matter what the nature of the plastic employed may be, and in particular no matter what its ability to crystallize may be, a temperature range for the rough shape always exists in which the subsequent blowing process leads to orientation of the macromolecular chains of the plastic and, consequently, to an often spectacular improvement in the mechanical properties of the products thus manufactured.
Numerous procedures leading to oriented final products have consequently been proposed, all of which involve a stage of thermally conditioning the rough shapes before blowing, which consists of bringing these rough shapes to within the desired temperature range by cooling and/or heating.
One technique for the manufacture of oriented hollow articles consists, in a first stage, or pre-blowing a portion of tubular parison extruded into a preform mold in which the preform thus molded is thermally conditioned in order to enable it to become oriented during final blow-molding, and then, in a second stage, of transferring the conditioned preform to a blow-mold where it acquires its final shape.
The amount by which the portion of parison is expanded during the pre-blowing process is relatively small, because it is preferable for the preform thus prepared to be stretched to a large extent during the subsequent blowing process, since only the latter process is carried out under conditions which lead to orientation of the molecules. Likewise, the length of the preform is less than the height of the finished article so that, during the blowing process, the preform is stretched both axially and radially and thus undergoes bidirectional orientation of the same magnitude in both directions.
In general, during the pre-blowing stage, a preform is thus produced, the diameter of which is only very slightly greater than that of the starting portion of tubular parison, since this expansion takes place to the detriment of the subsequent orientation. In practice, the increase in diameter during the pre-blowing process is kept at less than 20%, relative to the diameter of the starting portion of tubular parison.
Furthermore, the preform prepared by pre-blowing usually possesses a sealed and deflashed base. Hitherto, it has been regarded as satisfactory to produce preforms by pre-blowing, in which the shape of the base is reasonably similar to that of the final hollow article desired. However, it has been found that the subsequent blowing and stretching processes lead to oriented hollow articles in which some zones have been considerably thinned down. This is the case, especially, in the zone of the lateral surface near the base. The existence of these weakened zones causes practically all the benefit of the orientation to be lost, and thus has an extremely detrimental effect.
It is thus apparent that, hitherto, it has not been possible, by means of the pre-blowing and thermal conditioning technique, to manufacture oriented hollow articles which possess constant mechanical properties at all points. On the contrary, the oriented hollow articles produced by this technique always possess zones of weakness, especially in the vicinity of the periphery of their base.